An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

The radioactive transformation of a nuclide by alpha-particle emission. Also called alpha disintegration.

→ alpha; → decay.

The transformation of a → radioactive nuclide in which a → beta particle is emitted. In beta minus decay, a → neutron changes into a → proton, → antineutrino, and → electron: n → p + e + ν^-. Beta plus decay involves the conversion of a proton to a neutron, → positron, and → neutrino: p → n + e⁺ + ν.

Beta (β), from → beta particle; → decay.

→ beta particle.

→ beta particle; → minus; → decay.

→ beta particle.

→ beta particle; → plus; → decay.

The process when the energy difference between the excited and non excited states of an atom is taken away by an electron during a collision.

→ collisional; → decay.

In theoretical models, the hypothetical transformation of a → non-baryonic dark matter particle when symmetry is violated at special physical conditions. Dark matter decay and → dark matter annihilation are expected to produce enormous amounts of energy in the form of gamma-rays, cosmic rays, etc.

→ dark; → matter; → decay.

1a) To become decomposed.
1b) Of a radioactive nucleus, to disintegrate spontaneously into one or more different nuclei, accompanied by the emission of → alpha particles, → beta particles, → positrons, and/or → gamma rays.
2a) Decomposition.
2b) The → transition of a system from an → excited state to a less excited one. Radiative decay refers to the process when the energy difference between the states is taken away by radiation. Likewise, collisional decay is when the energy difference is removed by a target during a collision.
2c) Of a spacecraft, a gradual decrease in the radius of its orbit over time, caused by aerodynamic drag of the atmosphere and other forces. The rate of orbit decay rises as the spacecraft falls and encounters increasing atmospheric density, eventually resulting in reentry.
→ alpha decay; → beta decay; → dark matter decay; → decay chain; → decay constant; → decay mode; → decay product; → decay time; → inverse beta decay; → Ohmic decay time; → orbit decay; → radioactive decay; → turbulence decay.

From O.Fr. decair, from V.L. *decadere "to fall off," from L. cadere "to fall," PIE base *kad- "to fall" (cf. Pers. Gilaki katan "to fall," ba.ka.tam "I fell," dakatan "to fall (in a marsh, in a pit)," vakatan "to fall from tiredness, be exhausted," fakatan "to fall from (lose) reputation," Pers. Laki: katen "to fall," kat "he fell," beko "fall!," Pers. Tabari: dakətə "fallen," dakətən "to crash down," dakət.gu "stray cow," Arm. chacnum "to fall").

Tabâhidan, verbal form of tabâhi, noun form of tabâh "spoiled, ruined, destroyed," Mid.Pers. tapâh "spoiled, destroyed." Maybe related to Mod.Pers. tâb "affliction, pain, torment; heat, burning," tab "fever," tâbidan, tâftan "to shine," tafsidan "to become hot," Av. tāp-, taf- "to warm up, heat," tafsat "became hot," tāpaiieiti "to create warmth," cf. Skt. tap- "to spoil, injure, damage; to suffer; to heat, be/become hot," tapati "burns," L. tepere "to be warm," tepidus "warm," PIE base *tep- "warm."

A series of nuclear decays produced by successive → daughter products, when the daughters are themselves → radioactive. For example, the decay chain N₁→ N₂→ N₃→ ... in which the parent nuclide N₁ decays to the daughter N₂, which in turn decays to N₃. Each → radionuclide in the decay chain can → branch to more than one daughter.

→ decay; → chain.

A constant of proportionality occurring in the formula expressing spontaneous → decay of → radionuclides. The number of atoms decaying is given by N = N₀e^-kt, where N₀ is the number of nuclei in the given volume of the substance at instant t = 0, N is the number of nuclei at t, and k is decay constant. Decay constant is related to → half-life by τ = ln2/k, roughly 0.693/k.

→ decay; → constant.

A possible type of decay of a → radionuclide or → elementary particle. In general, a given particle may decay by more than one decay mode. Usually the number of decay modes is one or two. There are, however, → nuclides which have many decay modes; for example the nuclide ¹¹Li has seven decay modes.

→ decay; → mode.

A → stable nuclide or → radioactive nuclide formed by the → disintegration of a → radioactive isotope, either directly or as a result of a → decay chain. Also called → daughter product. For example, the decay product of ²³⁸U is ²⁰⁶Pb, after passing through the following chain: ²³⁸U → ²³⁴Th (4.5 billion yr) → ²³⁴Pa (24 days) → ²³⁴U (1 min) → ²³⁰Th (245,000 yr) → ²²⁶Ra (76,000 yr) → ²²²Rn (1,600 yr) → ²¹⁸Po (3.8 d) → ²¹⁴Pb (3 m) → ²¹⁴Bi (27 m) → ²¹⁴Po (160 microseconds) → ²¹⁰Pb (22 yr) → ²¹⁰Bi (5 d) → ²¹⁰Po (138 d) → ²⁰⁶Pb.

→ decay; → product.

The time required for the amplitude of a vibrating system to decrease to 1/e of its initial value.
Of an orbiting object, its lifetime in a non stable orbit.

→ decay; → time.

A type of → radioactivity in which some unstable atomic nuclei dissipate excess energy by a spontaneous electromagnetic process, usually accompanied by → alpha decay or → beta decay.

→ gamma; → decay.

A collision of a proton with an electron that produces a neutron and an electron neutrino. See → beta decay.

→ inverse; β, letter of Gk. alphabet; → decay.

An upper bound on the time scale on which the magnetic field of a system would decay in the absence of any other agent. It is expressed as: τ_μ = R² / μ, where R is the scale size of the system, η the magnetic diffusivity (η = 1 / μσ, where μ is the magnetic permeability and σ the electrical conductivity). For a star like the Sun, τ_μ  ≅ 10¹⁰ years, so a fossil magnetic field could survive for the star's lifetime on the main sequence. For the Earth, τ_μ  ≅ 10⁴ years, so a → dynamo is required to explain the persistence of the geomagnetic field.

→ Ohmic; → decay; → time.

A gradual change in the orbit of a spacecraft caused by aerodynamic drag of a planet's outer atmosphere and other forces.

→ orbit; → decay.

A phase in the → light curve evolution of eruptive objects such as → dwarf novae, → Soft X-ray Transients, and transient → magnetars which follows the characterized sudden increase in their flux (over a factor ~ 1000 over the quiescent level). Outburst decay is slow and lasts months or years.

→ outburst; → decay.

The process when the energy difference between the excited and non excited states of an atom is taken away by radiation.

→ radiative; → decay.

Spontaneous emission by a nucleus of photons or particles.

→ radioactive; → decay.

The process whereby turbulence evolves by exchanging energy, leading to → dissipation.

→ turbulence; → decay.